What is the difference between Unwrap and Smart uv project?
"Unwrap" is when Blender will determine where each and every face should be located on your UV Image Editor, For example, If I were to unwrap an entire cube or sphere, it would look something like this:
As you can see, all of the faces of this sphere are stacked upon one another, fitting the entire frame of each face with the texture. But all of this can be solved if you Mark Seams on your object.

Smart UV Unwrap is when if you were to chose this option, you would be given 4 options to change and edit the way blender is going to analyze your mesh unwrapping;Angle Limit, Island Margin, Area weight, and Correct Aspect. You can tweak these options to see what you would prefer most and what you like best.
Ex.
In my opinion, I would go for the "Unwrap" because if you "Mark Seams" you can control the way blender unwraps your mesh in a much easier way instead of having to work with all those options in "Smart UV Unwrap."